This invention relates to a device for eliminating the curling of a copy which is formed by an image forming device such as an electronic copying machine, and more particularly to a device in which a roll member small in diameter is pushed against an endless belt, to forcibly bend a curled sheet in the direction opposite to the direction of curling, thereby to straighten the sheet (hereinafter referred to as "a sheet straightening device", when applicable).
In forming a copy with an image forming apparatus of electrophotographic system such as an electronic coping machine, a method is employed in which a recording sheet bearing a toner image is delivered to a fixing unit, where the toner image is fixed by heat and pressure. The copy thus formed may be greatly curled depending on the density of the toner image formed on the sheet, and the kind and thickness of the latter. If the sheet thus curled is delivered into a sheet discharging tray as it is, sometimes it is jammed being caught in the sheet conveying path. Furthermore, in the case where an after-treatment device such as a sorter is provided beside the sheet discharging section of the image forming apparatus, the curled sheet results in a difficulty that, in the sorter, the number of copies received in the tray is decreased; that is, the copy accommodating capacity of the sorter is decreased. In an automatic both-side copying machine, the curled sheet may cause the following difficulties: That is, when a sheet on one side of which an image has been formed is conveyed for formation of an image on the other side, it may be jammed in the sheet conveying path, or it may be inaccurately delivered into the intermediate tray, thus impeding the formation of the image on the other side of the sheet.
In order to straighten the curled sheet, a method has been proposed in the art in which, as disclosed, for instance, by Japanese Utility Model Application (OPI) No. 97352/1990 (the term "OPI" as used herein means an "unexamined published application"), a curling eliminating device is arranged in a sheet conveying path through which a sheet on one side of which an image has been formed is conveyed to the intermediate tray. In this conventional device, the sheet conveying path is bent greatly, and a metal roller relatively small in diameter and an elastic roller relatively large in diameter are arranged at the bend of the sheet conveying path in such a manner that those rollers abut against each other, so that the sheet is abruptly bent when passing between the two rollers, and is then caused to slide along the curved portion of a guide board. And, two sets of the above-described curling eliminating devices are provided so as to bend a curled sheet in opposite directions, whereby the curled sheet can be straightened on one side of which an image has been formed.
A monochromatic (black and white) copy formed with an ordinary image forming apparatus is relatively low in image density, and therefore the curling of a sheet which is caused when the sheet passes through the fixing unit is, in general, relatively little. Hence, in this case, the curled sheet can be readily straightened with the above-described curling eliminating device. On the other hand, in the case where, as was described above, it is required to set the curling eliminating device between the fixing unit and the sheet discharging tray, it may be rather difficult to do so because the limitation in occupying space of the device. In the case of a color copy high in image density, it is liable to be greatly curled, and the curling cannot be eliminated with the above-described curling eliminating device.
In order to eliminate the curling of such a color copy, a sheet straightening device as disclosed, for instance, by Japanese Patent Application (OPI) No. 167450/1984 may be employed. In this conventional sheet straightening device, a depressing roll is provided for a belt adapted to convey a copy. The depressing roll is pushed against the belt to bend the latter, so that a sheet is forcibly bent when passing between the belt and the depressing roll thus pushed against the latter, thereby to eliminate the curling of the sheet. The depressing roll is so designed that it can be fixed or rotated. That is, the predetermined front end portion of the curled sheet is rubbed with the depressing roll fixed, thereby to eliminate the curling.
In the conventional sheet straightening device, the force of depression applied through the depression roll to the belt is set to a predetermined value. Therefore, if the belt is bent greatly so as to handle a color copy which is liable to be greatly curled, then a black and white copy may be curled in the opposite direction. In the case where the sheet straightening ability is set for a copy low in image density, then the device cannot sufficiently handle a color copy, and cannot satisfactorily eliminate the curling of a sheet depending on the thickness and paper quality of the sheet. In addition to the above-described difficulties, in the case where color images are formed on both sides of a sheet, the following troubles may be involved. That is, if the color images on both sides of the sheet are different in image density, then the sheet is curled in a plurality of directions; that is, the curling of the sheet is not constant in direction. Thus, even if the conventional sheet straightening device is applied to the curled sheet on both sides of which color images have been formed, it is rather difficult to eliminate the curling of the sheet with high efficiency.